1. Field of the invention
This invention relates to an information recording medium having an air-sandwich structure, and more particularly to a recording medium having an air-sandwich structure for writing and/or reading information by means of laser beam of high energy density.
2. Description of prior Arts
Information recording media utilizing a beam of high energy density such as laser beam have been developed in recent years and are put to practical use. Such recording medium is generally called "optical disc", and the practical applications thereof have been found, for example, as video disc and audio disc as well as disc memory for large-capacity computer and large-capacity static image file.
The optical disc basically comprises a disc-shaped transpartent substrate of a plastic or glass material and a recording layer composed of a metal or a semi-metal such as Bi, Sn, In or Te which is provided on the substrate. Further, a subbing layer of a polymer material is generally provided between the substrate and the recording layer so that the adhesion between the substrate and the recording layer is ensured and the sensitivity of the optical disc is enhanced.
Writing of information on the optical disc can be conducted, for example, by irradiating the optical disc with a laser beam. Under irradiation with the laser beam, the irradiated area of the recording layer of the optical disc absorbs the energy of beam and a rise in temperature locally occurs and as a result, a chemical or physical change is caused to alter optical characteristics of the recording layer in the irradiated area, whereby the recording of information can be made. Reading of information from the optical disc is also conducted by irradiating the optical disc with laser beam. The information can be reproduced by detecting reflected light or transmitted light corresponding to the alteration in the optical characteristics of the recording layer.
As a disc structure for protecting the recording layer, there has been recently proposed an air-sandwich structure in which a recording layer is provided on at least one of two disc-shaped substrates and positioned inside the two substrates opposed to each other at a given space by interposing a ring-shaped inner spacer and a ring-shaped outer spacer between the substrates so as to form space enclosed with the two substrates and the two spacers. In the optical disc having such a structure, the recording layer is not directly exposed to the outer air and the recording and reproduction of information is conducted with laser beam passing through the substrate. Accordingly, in general there is no possibility that the recording layer is physically or chemically damaged or the surface thereof is covered with dust which is to interfere with the recording and reproduction of information.
The conventional information recording medium of air-sandwiched structure generally has an appearance illustrated in FIG. 1 and a section schematically illustrated in FIG. 2.
In FIG. 2, each of two disc-shaped substrates 21a, 21b has a subbing layer 22a, 22b and recording layer 23a, 23b on the surface, respectively. The two substrates are combined with each other via a ring-shaped inner spacer 25 having adhesive layers 24a, 24b on the upper- and under-sides and a ring-shaped outer spacer 27 having adhesive layers 25a, 25b on the upper- and under-sides. Thus, an inner space 28 enclosed with the two substrates 21a, 21b and the two spacers 25, 27 is provided.
As described above, the substrates and the spacers are combined with each other by an adhesive. In the conventional recording medium of air-sandwich structure, the adhesive, its components or its solvent is liable to enter the enclosed inner space and accordingly to reduce the sensitivity and durability of the medium. In more detail, the recording medium of air-sandwich structure is generally constructed to have a high air-tightness. For giving a satisfactory air-tightness to the structure, an excessive amount of an adhesive is supplied between the substrate and the spacer. A portion or most of an excess of the supplied adhesive is accordingly pushed out into the enclosed inner space. Thus introduced adhesive may contact directly the recording layer to deteriorate the characteristics of the recording layer. Otherwise, the introduced adhesive is decomposed in the period of storage or in the writing or reading operation using a laser beam of high energy. The decomposition products may contact the recording layer to deteriorate chemically the recording layer.
Other problem resides in entrance of a solvent of the adhesive. In a certain process for the preparation of the recording medium of air-sandwich structure, an adhesive in a solvent is applied on the substrates and/or the spacers, and the substrates and the spacers are immediately combined and then allowed to stand for drying the adhesive layer. In this process, a portion of the solvent inevitably enters the enclosed inner space. The solvent or its vapor may contact the recording layer to chemically deteriorate the recording layer. Further, the vapor of the solvent may dissolve a part of the formed adhesive layer upon continuous contact with the adhesive layer and accordingly to reduce the air-tightness of the air-sandwich structure.
Moreover, the conventional recording medium of FIG. 2 is sometimes constructed disadvantageously to give variation of thickness within the combined unit. The thickness of the recording medium of air-sandwich structure is defined by the total thickness of the subtrates, the spacer and the adhesive layers. The thickness of the adhesive layer varies with variation of pressure applied thereto in the manufacturing process as well as variation of the drying conditions. Accordingly, the thickness of the adhesive layer sometimes varies within the combined unit. The disc of thickness varying locally does not show a satisfactorily flat plane threon and is liable to vibrate in the vertical direction to the disc plane in the rotatory action, possibly resulting in occurrence of focus-out.
As a proposal for solving the above-stated problems of the conventional recording medium of air-sandwich structure, Japanese Patent Provisional Publication No. 58(1983)-64651 teaches a medium using a spacer which has concaves in the areas to contact the substrates, the concave being capable of receiving an adhesive. This prior art is though to effectively solve the above-stated problems as far as the adhesive is employed in an amount precisely corresponding to the inner volume of the concave. However, if the adhesive is employed in an excessive amount to render the adhesion and sealing between the substrate and the spacer complete, the excess portion is inevitably pushed out to enter the enclosed inner space in the same manner as occurring in the conventional recording medium. Further, the spacer structure of this prior art cannot be adopted when the preparation process using an adhesive of the adhesive-in-solvent type is employed.